Method and apparatus for rapidly extracting significant data from a sparse object

ABSTRACT

A control circuit for an apparatus for inspecting objects, such as glass bottles and the like, for defects includes an interface circuit connected between a source of data signals and means for processing information obtained from the object. The interface circuit receives the data signals, typically in digital series form, and includes a latch for storing one of the digital signals, a pair of adders, and a storage means for a plurality of threshold signals. Each data signal is compared to the preceding data signal stored in the latch in one of the adders to generate a difference signal representing the difference in magnitudes between the two signals. Each difference signal is compared with a selected one of the stored threshold signals in the other adder to generate an event signal representing the difference in magnitudes between the two signals. The means for processing information includes a pair of control units and a master control means for alternately connecting the control units to the interface means, whereby one of the control units is receiving a group of the event signals representing one object while the other control unit is processing a preceding group of the event signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to sidewall inspection devicesfor containers and in particular to a method and apparatus forextracting significant data with respect to defects from a sparseobject, such as a glass bottle.

2. Description of the Prior Art

The use of optical scanning devices for inspecting the sidewalls ofcontainers is well known. Numerous devices, such as those shown in U.S.Pat. Nos. 3,708,680 and 3,716,136, have circuitry including means forreceiving and interpreting light passed through or directed onto an itemunder inspection. Such devices incorporate either a visual display forcomparison of the item or employ a device capable of producing aresistance proportional to the intensity of light directed thereon.Whether the output of such a device is visual or electrical in nature,it is eventually compared against a model to determine if the item underinspection is suitable as to size and construction and is without flaws,cracks, or foreign objects. Such devices are each intended to provide anautomated inspection means for checking, as in a moving column ofbottles, single or multiple objects in that moving column.

U.S. Pat. No. 3,877,821 discloses an apparatus having a scanning arraythat is serially interrogated to generate a train of pulses havingamplitudes representing the light transmitted through an object underinspection. Adjacent pulses are compared to generate pulses havingamplitudes which represent the difference in pulse amplitudes. Thedifference pulses can be utilized to indicate a defect in the objectbeing inspected. U.S. Pat. No. 3,942,001 discloses an apparatus fordetecting the presence of extraneous matter or cracks in translucentcontainers. A spot beam of light is projected through the container togenerate an inspection signal which is compared with an acceptancesignal. The acceptance signal amplitude is varied in accordance with theposition of the spot beam with respect to the container.

SUMMARY OF THE INVENTION

The present invention concerns an apparatus for extracting only thesignificant data from an optical inspection of a sparse object, such asa glass bottle. A digitized video signal representing a point ofinspection is generated by a camera and associated light source to aninterface circuit including an adder and a latch in which is stored thedigitized video signal from the previous point of inspection. A signalrepresenting the difference between the present digitized signal and thestored digitized signal from the latch is generated by the adder andcompared with a stored threshold level for the current point ofinspection. If the threshold level is exceeded, an event signal isgenerated and stored in the interface circuit.

After the object has been scanned, the group of event signals isprocessed to determine if a defect is present. A master control unitmeans alternately connects a pair of control units to the interface,whereby one of the control units is receiving a group of event signalswhile the other control unit is processing a preceding group of eventsignals.

It is an object of the present invention to provide a means andapparatus for rapidly extracting significant data from a sparse object,such as a glass bottle.

It is another object of the present invention to rapidly extractsignificant data in a manner suitable for computer analysis.

It is a further object of the present invention to provide a sidewallinspection device that can be easily integrated with an existingapparatus for moving transparent or translucent items past an inspectionpoint, automating the inspection process thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an apparatus for detecting defects inobjects according to the present invention; and

FIG. 2 is a block diagram of the inspection device interface of theapparatus for detecting defects of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, there is illustrated in FIG. 1 a blockdiagram of an apparatus for detecting defects in objects according tothe present invention. An object, such as a glass bottle (not shown), isscanned by a camera 10. The camera 10 generates a plurality of signalsproportional in magnitude to the amount of light received from the glassbottle. In the preferred embodiment of the invention, a light source(not shown) directs a beam of light through the glass bottle underinspection and into the camera 10. The camera 10 includes a plurality ofphotosensitive devices, such as photodiodes, which are verticallyarranged in a linear array. It has been found that a linear array of twohundred fifty-six photodiodes yields statisfactory results. Thephotodiode is a variable resistance device that will pass a voltageproportional to the amount of light falling thereon. Each photodiodereceives light which has passed through different segments or portionsof the bottle under inspection. If a flaw, crack, or foreign object iscontained in the bottle, then the light passing through that portion ofthe bottle will be partially blocked or reflected and the correspondingphotodiode will register a lesser intensity of light than had no defectbeen present.

The signals from the photodiodes of the camera 10 are supplied to asampler 14 on a plurality of lines 12. Each of the photodiodes issampled in sequential order, producing a series of pixel signals on aline 16 which represent the amount of light which passed through thebottle under inspection along one vertical sequential sweep of thephotodiodes. The sampler 14 is a device well known in the art. Byrotating the bottle under inspection relative to the camera 10, aplurality of different sweeps can be made, each sweep inspecting adifferent portion of the bottle. It has been found that about threehundred seventy-five to four hundred sweeps will sufficiently cover anaverage bottle and insure an accurate inspection. Thus, the sampler 14generates a plurality of series of pixel signals on line 16 representingthe amount of light passing through the inspected portions of the entirebottle.

The pixel signals from the sampler 14 on line 16 are an input to aninspection device interface 18. The interface 18 rapidly extractssignificant data from a sparse object, such as a glass bottle, in amanner suitable for computer analysis. When a bottle is ready to bescanned, the interface 18 is enabled to receive and store dataconcerning that bottle. When no bottle is ready to be scanned, theinterface 18 stores the data concerning the last scanned bottle until anew bottle is ready to be scanned. The operation of the interface 18 ismore fully explained below.

The interface 18 is a means for generating groups of signalsrepresenting the characteristics of the bottle under inspection. Theoutput of the interface 18 is fed to a control circuit for generating areject signal whenever a defective bottle is detected. The controlcircuit includes a first control unit means 20 and a second control unitmeans 22, which receive the output signals from the interface 18 overlines 24 and 26 respectively. The first control unit 20 and the secondcontrol unit 22 are each responsive to the groups of signalsrepresenting the characteristics of the bottles under inspection fordetermining whether to generate a reject signal.

The first control unit 20 and the second control unit 22 are connectedto a master control unit means or processor 28 by lines 30 and 32respectively. The master processor 28 also provides inputs to theinterface 18 over a plurality of lines 34 to allow an operator to setcertain tolerance limits, as will be more fully described below. Themaster processor 28 alternately connects one of the first and secondcontrol units 20 and 22 to the interface 18 to receive groups of signalsrepresenting the characteristics of a bottle while the other of thefirst and second control units 20 and 22 determines whether to generatea reject signal based upon the plurality of signals representing thecharacteristics of a preceding bottle. Thus, while the first controlunit 20 is reading data from the inspection interface 18 concerning abottle which has just been scanned, the second control unit 22 isprocessing data obtained on a prior scan to determine whether togenerate a reject signal for the preceding bottle.

The master processor 28, the first control unit 20, and the secondcontrol unit 22 can all be microprocessors, such as a model 6800manufactured by Motorola which is conventional and well known in theart. The master processor 28 has an input device 36 by which an operatorcan program the system and set various tolerance parameters. The inputdevice 36 is connected to the master processor 28 by a line 38. Themaster processor 28 is also connected by a line 40 to an output device42, such as a video display, so as to permit an operator to monitor orcalibrate the system. Alternatively, the device 42 can be a meansresponsive to a reject signal generated by the master processor 28 forrejecting a particular bottle which has been determined to be defective.A further input to the master processor 28 is a gauge 44. The gauge 44is provided to generate a signal on a line 46 when a bottle is in theproper position to be scanned.

The interface 18 can receive data so long as the gauge 44 signals that abottle is in the proper scanning position. When the gauge 44 ceases togenerate such a signal, as during the period when the inspected bottleis removed and an uninspected bottle is moved in, the collectedinformation is stored in the interface 18. The master processor 28prevents interference between the first and second control units 20 and22 by selecting one of the units to receive the data held in theinterface 18. When all of the data has been transferred to the firstcontrol unit 20, for example, the interface 18 is free to receive newdata on the next bottle as soon as the signal from the gauge 44 isrestored. The first control unit 20 processes the data in order todetermine whether to generate a reject signal. When scanning iscompleted on the next bottle and the gauge 44 ceases to generate itssignal, the accumulated data is stored in the interface 18. The masterprocessor 28 then selects the second control unit 22 to receive the datawhile the first control unit 20 continues to process the originalinformation. Thus, each of the control units 20 and 22 has two fullcycles of the gauge 44 to process the data concerning each bottle todetermine whether or not to generate a reject signal. By providingparallel processing paths, the control circuit increases the speed andefficiency of the inspection apparatus.

Referring now to FIG. 2, there is illustrated a block diagram of thedetails of the inspection device interface 18. The interface 18 is ameans for rapidly extracting significant data from a sparse object, suchas a glass bottle, in a manner suitable for computer analysis. Thesampler 14 can generate digital signals, or analog signals to ananalog-to-digital converter, representing the magnitude of the lightreceived by the camera 10. Line 16 presents the plurality of signals toan event detector 48 including a data latch 50 and an adder 52. Thelatch is a means for storing one of the plurality of signals. In theillustrated embodiment, the preceding pixel signal is stored in thelatch 50 and is presented to the complementary input of adder 52. Thus,the adder 52 is a means for generating a signal which represents thedifference between the magnitude of the stored preceding pixel signal inthe latch 50 and the successive pixel signal presented on line 16. Theoutput of the adder 52 is a signal representing the difference in themagnitudes of adjacent pixel signals. When the difference signal isgenerated by adder 52, the present pixel signal is stored in latch 50 tobe compared with the next pixel signal. A control logic unit 54 of theinterface 18 generates a command over a LATCH NEXT PIXEL line to causethe latch 50 to store the present pixel signal available on line 16. Thecontents of the latch 50 can be cleared to zero by a command from themaster processor 28 over a CLEAR L line.

The difference signal from the adder 52 can be either positive ornegative, depending upon the magnitudes of the present and previouspixel signals. Because only the magnitude of the difference betweenadjacent pixel signals is relevant in the detection of defects, it isconvenient to feed the difference signal to a means for generating theabsolute magnitude of the difference signal. As illustrated, the outputfrom adder 52 is fed to an absolute magnitude circuit 56. The circuit 56can be constructed of a plurality of exclusive OR gates, as is wellknown in the art. The CARRY output of adder 52 controls the absolutemagnitude circuit 56 such that the output is always positive.Rectification of the difference signal prevents misleading comparisonreadings in the event detector 48.

The event detector 48 includes a means for storing a threshold signal.In the preferred embodiment, a threshold random access memory (RAM) 58is provided for storing a plurality of threshold signals. Each thresholdsignal stored in the threshold RAM 58 corresponds to a specific pixeldifference signal generated by the adder 52. The means for selecting theindividual threshold signal from the threshold RAM 58 which correspondsto the present difference signal is a diode counter 60. The diodecounter 60 can be cleared to zero by a command from the control logic 54over a CLEAR DC line and can be incremented by a command over anINCREMENT DC line. The diode counter 60 provides the threshold RAM 58with the memory address of the proper threshold signal. The desiredthreshold signals can be loaded into the threshold RAM 58 from themaster processor 28 over a LOAD DATA line. The output of the diodecounter 60 is also connected to an internal data bus 62.

The signal from the threshold RAM 58 is presented to the complementaryinput of an adder 64 where it is combined with the signal from theabsolute magnitude circuit 56. The adder 64 is a means for generatingevent signals when the difference signal obtained from the absolutemagnitude circuit 56 differs from the threshold signal obtained from thethreshold RAM 58. Event signals are generated, over an EVENT line to thecontrol logic 54, indicating the detection of a defect, and over aMAGNITUDE line to the internal data bus 62, indicating by how much thedifference signal differed from the threshold signal.

Upon receiving a signal from the gauge 44 that a bottle is ready to bescanned, the master processor 28 generates a signal over a GAUGE line tothe control logic 54. In response to that signal, the control logic 54generates a signal over a CLEAR SC line to a sweep counter 66. Thecontents of the sweep counter 66 are thus cleared to zero before eachbottle is scanned. The output of the sweep counter 66 is connected tothe internal data bus 62.

To initiate a sweep, the mater processor 28 generates a signal over aSTART SWEEP line to the control logic 54. In response to that signal,the control logic 54 increments the sweep counter 66 by generating asignal over an INCREMENT SC line. The control logic 54 also clears thecontents of the diode counter by generating a signal over the CLEAR DCline. The control logic 54 further generates a signal over a CLEAR ECline to clear an event counter 68. These three initialization functionsprepare the interface 18 for the receipt of data. The output of theevent counter 68 is connected to the internal data bus 62. The eventcounter 68 generates a signal on an OVERFLOW line to the data bus 62when the contents of the register exceed its limits. The event counter68 is incremented by the control logic 54 over an INCREMENT EC line eachtime that the event detector 48 signals that an event has occurred.

The interface 18 includes a means for storing the event signals. Aninterface random access memory (RAM) 70 is provided for reading andstoring the signals available on the data bus 62. The first control unit20 and the second control unit 22 alternatively read the accumulateddata from the interface RAM 70 through the data bus 62 and lines 24 and26 respectively. Data is stored in the interface RAM 70 when the controllogic 54 generates a signal over a WRITE line. The interface RAM 70 alsogenerates a signal on an OVERFLOW line to the data bus 62 when thecontents of the register exceed its limits. A RAM counter 72 providesthe interface RAM 70 with memory address locations. The RAM counter 72can be cleared to zero by a command from the control logic 54 over aCLEAR RC line and can be incremented by the control logic 54 by acommand over an INCREMENT RC line.

The interface 18 also includes a means for defining a range forextracting data. In the illustrated embodiment, a window generator 74 isprovided to limit the number of sweeps over which data can be extracted.A lower sweep limit is entered by an operator through the input device36 to the master processor 28. The instruction is sent over a LO SETline to a low sweep comparator 76. The output of the sweep counter 66 isalso an input to the low sweep comparator 76. When the number in thesweep counter 66 equals or exceeds the number generated over the LO SETline, the low sweep comparator 76 generates a signal over a SET line toa flip-flop 78. The flip-flop 78 generates a signal over an ENABLE lineto the control logic 54, instructing it to process the incoming data.Signals received by the interface 18 on sweeps taken of a bottle belowthe lower sweep limit are ignored to prevent erroneous data associatedwith the initial sweeps from being processed. Similarly, the operatorcan enter a high sweep limit value to cause the interface 18 to stopprocessing data after a certain number of sweeps. The master processor28 sends the instruction over a HI SET line to a high sweep comparator80. The output of the sweep counter 66 is also an input to the highsweep comparator 80. When the number in the sweep counter 66 equals orexceeds the number generated over the HI SET line, the high sweepcomparator 80 generates a signal over a RESET line to the flip-flop 78.The flip-flop 78 thus ceases to generate the signal over the ENABLEline, causing the control logic 54 to ignore all subsequent data.

Prior to utilizing the apparatus for detecting defects, the operatorwill enter the parameters under which the machine will operate throughthe input device 36. The parameters include the low and high sweeplimits and the group of threshold signals. The low and high sweep limitsdefine the sweep window, which is the range of sweeps over which datacan be accepted by the interface 18. By selecting a particular set ofthreshold signals to be loaded into the threshold RAM 58, the operatordetermines the acceptable tolerances of light deviation which will causean event to be detected. The master processor 28 loads the appropriatedata into the interface 18.

When a bottle has been moved into a proper position for scanning, thegauge 44 generates a signal to the master processor 28. The signal isrelayed along the GAUGE line to the control logic 54, which generatessignals to clear the contents of both the sweep counter 66 and the RAMcounter 72. These tasks are performed each time a new bottle is ready tobe inspected. The interface 18 is then prepared to receive data from thecamera 10.

At the beginning of each sweep, the master processor 28 generates asignal over the START SWEEP line to the control logic 54. The controllogic 54 generates appropriate signals to clear the contents of thediode counter 60, clear the contents of the event counter 68, andincrement the contents of the sweep counter 66. These tasks areperformed at the beginning of each sweep made by the sampler 14.

The incoming pixel signals are fed to the adder 52 and the latch 50. Thelatch 50 holds the previous pixel signal at its output, which is thenfed to the complementary input of the adder 52. Thus, the output of theadder 52 represents the difference between the two adjacent pixelsignals. The output of the adder 52 is fed to the absolute magnitudecircuit 56, which insures that the input to adder 64 is always apositive signal.

The threshold RAM 58 holds the programmed plurality of thresholdsignals, each of which corresponds to a specific difference signalrepresenting a pair of pixels. Since each pixel signal represents asampled photodiode in the camera 10, the diode counter 60 can beincremented with each incoming pixel signal to select the memory addressof the appropriate threshold signal stored in the threshold RAM 58. Thatparticular threshold signal is fed to the complementary input of adder64 to be compared with the actual difference signal generated by adder52 and rectified by the absolute magnitude circuit 56. The output ofadder 64 is a plurality of event signals which represent a comparisonbetween the difference signal and the threshold signal. When themagnitude of the difference signal exceeds a predetermined amount, theadder 64 will generate an event signal over the EVENT line to thecontrol logic 54. The magnitude of the event signal as well as theoutput of the diode counter are gated onto the data bus 62 for storagein the interface RAM 70.

When an event is detected during the sweep window, as defined by theoperator using the window generator 74, the control logic 54 generatessignals which increment the event counter 68 and increment the RAMcounter 72. The control logic 54 also generates a signal over the WRITEline to the interface RAM 70 to read and store the contents of the diodecounter 60 and the magnitude of the output adder 64. This process isrepeated with each pair of adjacent pixel signals until a sweep iscompleted. The signal on the START SWEEP line is removed at the end ofeach sweep, causing the contents of the sweep counter 66 and the eventcounter 68 to be written into the interface RAM 70 if one or more eventshave occurred in that particular sweep. Thus, in each sweep where anevent is detected, the gathered data includes a series of events denotedby diode number and event magnitude, followed by a final single entryconsisting of the sweep number and the number of events which occurredin that sweep. When the next sweep of the same bottle begins, thecontents of the diode counter 60 are cleared to zero, the contents ofthe event counter 68 are cleared to zero, and the sweep counter 66 isagain incremented. The scanning continues until the window generator 74disables the interface 18 when the high sweep limit has been reached.

The groups of signals stored in the interface RAM 70 which represent thecharacteristics of the inspected bottle are then fed to either the firstcontrol unit 20 or the second control unit 22, as determined by themaster processor 28. The data in the interface RAM 70 is downloaded intothe selected control unit, which determines whether or not to generate areject signal for that particular bottle. Two checks are made beforeprocessing begins to make sure that the interface 18 has not overflowedbecause of an unusually bad bottle. These checks are indicated by statusflags on the event counter 68 and the interface RAM 70. If the contentsof either unit exceeds the capability of the register, a signal isgenerated over the resepctive OVERFLOW lines. When either overflowsignal is present, the bottle will be immediately rejected because of agross defect.

As stated above, the format of the data which is read by the selectedcontrol unit includes a series of diode numbers and associated eventmagnitudes, followed by a sweep number and a number of events. Thebottle data is downloaded from the interface RAM 70 to the particularcontrol unit. By checking each event along a sweep to see if it can belinked to a preceding event, the control units 20 or 22 can generate astring. A string is defined as a collection of one or more events inproximity to each other and having four properties which are calculatedduring generation. These properties include: the beginning of thestring, which is the first diode number; the end of the string, which isthe last diode number; the magnitude of each string, which is the sum ofthe magnitudes of each event comprising the string; and the number ofevents that formed the string. Checking for excess string magnitudeoccurs during string generation and the decision process will halt if astring magnitude exceeds a user-adjustable threshold. In other words,the selected control unit 20 or 22 links together events within a singlesweep to determine if the sum of the magnitudes of the events exceeds auser-specified tolerance. If so, a reject signal is generated and theparticular bottle will be removed.

If string checking does not reject the bottle, another processing stageis entered wherein the strings are checked to see if they form blobs. Ablob is defined as collection of strings in proximity to each other. Thestring diode numbers must overlap, or at most be within a user-specifiedrange, for the end of one string on one sweep and the beginning ofanother string on a different sweep. A blob has three properties whichare calculated during formation. These properties include blob width,blob magnitude, and the number of events in the blob. During blobformation, blob width and blob magnitude are checked againstuser-specified tolerances and processing stops if either threshold isexceeded. If a bottle is not rejected because of blob width or blobmagnitude, the number of events contained in the blob is compared toanother user-specified number. If the number of events exceeds thespecified tolerance, the bottle will also be rejected. If the bottle hasnot been rejected for any of the above reasons, it is considered a goodbottle and no reject signal will be generated.

The apparatus for detecting defects can also be utilized to generate anddisplay a picture of the object under inspection. A bottle is inspectedunder the normal procedure described above and data is stored in theinterface RAM 70. When the bottle has been completely scanned, themaster processor 28 instructs either the first control unit 20 or thesecond control unit 22 to receive the data from the inspection interface18. The selected control unit 20 or 22 does not process the receivedinformation but rather transmits the data in raw form to the masterprocessor 28. The gathered data includes the diode number, the sweepnumber, and the event magnitude for each event detected by the interface18. The data is then presented to the output device 42, which caninclude a two-dimensional graphic module and a video screen. The graphicmodule and video screen are well known in the art. The data can bedisplayed in a two-dimensional graphic form, utilizing the sweep numberof each event as the horizontal component and the diode number of eachevent as the vertical component. The video screen will display a dot ateach sweep and diode number location where an event was detected. Theresult is a two-dimensional representation of the inspected bottleshowing all of the detected defects, as if the bottle had been cutthrough one side and unwrapped for display. The event magnitude may beused in conjunction with a synthetic threshold level which can be variedto generate new pictures which show the effect that different thresholdlevels have. Using the apparatus in this mode, an operator is aided indetermining what the appropriate threshold levels for the particularstyle of bottle should be. Although the preferred embodiment of theinvention provides only a two-dimensional representation of the objectunder inspection, it will be appreciated that a three-dimensionalrepresentation could be generated on the video screen by the use ofadditional circuitry. Such circuitry is also well known in the art.

The apparatus for detecting defects can also be utilized to monitor thevideo output of the line scan camera. Such a use permits an operator tocalibrate the interface 18 without requiring the use of an oscilloscope.When the apparatus is operated in this mode, the master processor 28continuously clears the contents of the latch 50 to zero by generating asignal over the CLEAR L line. With the latch 50 cleared, the pluralityof pixel signals on lines 16 from the sampler 14 pass through the adder52 unaltered. The master processor 28 also utilizes the LOAD DATA lineto load the threshold RAM 58 with all zeros. Thus, every pixel signal isdetected as an event and is stored in the interface RAM 70. Since theinterface RAM 70 is limited in size, only one sweep of the bottle istaken to prevent memory overflow. The master processor 28 selects eitherthe first control unit 20 or the second control unit 22 to receive thedata from the interface RAM 70. The data includes the diode number andevent magnitude for each pixel of the sweep. The data is transferredfrom the selected control unit 20 or 22 to the master processor 28. Themaster processor 28 relays the information to the output device 42,which again can consist of a two-dimensional graphic module and a videoscreen. The graphic module can utilize the diode number as thehorizontal component and the event magnitude as the vertical component.The graph which is thus displayed on the video screen represents theamount of light received by the photodiodes over a single sweep. Theprocedure can be repeated continuously to simulate an oscilloscope.However, unlike an oscilloscope, no sweep or gain adjustments arenecessary since the data is always properly scaled to a specific diodenumber or event magnitude. Operation of the apparatus in this modepermits an operator to make sensitivity adjustments relating to theevent magnitude voltage without requiring the use of an oscilloscope.

In accordance with the provisions of the patent statutes, the principleand mode of operation of the invention have been explained andillustrated in its preferred embodiment. However, it must be understoodthat the invention may be practiced otherwise than as specificallyillustrated and described without departing from its spirit or scope.

What is claimed is:
 1. In an apparatus for detecting defects insuccessive objects and for providing a plurality of digital signals eachrepresenting the magnitude of light received from a corresponding pointon an object, an interface circuit comprising:means for storing thedigital signals; means for generating for each one of the digitalsignals a difference signal representing the difference in magnitudesbetween said one digital signal and a preceding one of the digitalsignals; means for storing a plurality of different threshold signals,each associated with the location of a different corresponding point onthe object; and means for comparing each of said difference signals witheach of said threshold signals at corresponding points on the object andgenerating an event signal when the magnitude of said difference signaldiffers from the magnitude of said corresponding threshold signal. 2.The interface circuit according to claim 1 wherein said means forstoring said event signal includes a random access memory.
 3. Theinterface circuit according to claim 1 further comprising means forstoring the event signal.
 4. The interface circuit according to claim 1or 3 wherein said means for generating said difference signal includesmeans for generating said difference signal as the absolute magnitude ofsaid difference signal.
 5. The interface circuit according to claim 1 or3 wherein said means for storing the digital signals includes a latchfor storing said preceding one of the digital signals.
 6. The interfacecircuit according to claim 1 or 3 wherein said means for generating saiddifference signal includes an adder having one input connected to thesource of digital signals and a complementary input connected to anoutput of said means for storing the digital signals for generating saiddifference signal.
 7. The interface circuit according to claim 1 or 3wherein said means for comparing includes an adder having one inputconnected to an output of said means for generating said differencesignal and a complementary input connected to an output of said meansfor storing a plurality of threshold signals for generating said eventsignal.
 8. The interface circuit according to claim 1 or 3 wherein saidmeans for storing a plurality of threshold signals includes a randomaccess memory.
 9. The interface circuit according to claim 8 includingmeans for selecting one of said threshold signals associated with thelocation of a corresponding point on the object.
 10. A method ofinspecting successive objects for defects by processing a plurality ofdigital signals, each representing the magnitude of light received froma corresponding point on an object, comprising the steps of:(a) storinga plurality of different threshold signals each associated with thelocation of a different corresponding point on the object; (b)generating for each one of the digital signals a difference signalrepresenting the difference in magnitudes between said one digitalsignal and a preceding one of the digital signals; (c) comparing each ofsaid difference signals with each of said threshold signals atcorresponding points on the object and generating an event signal whenthe magnitude of said difference signal differs from the magnitude ofsaid corresponding signal; and (d) storing said event signals.
 11. Themethod according to claim 10 including a step of storing each of thedigital signals as said preceding one until said difference signal isgenerated.
 12. The method according to claim 10 wherein said step (b)includes generating said difference signals as the absolute magnitude ofsaid difference signals.
 13. The method according to claim 10 whereinsaid step (b) is performed by combining said one digital signal and saidpreceding one of the digital signals in an adder.
 14. The methodaccording to claim 10 wherein said step (c) is performed by combiningsaid difference signal and said corresponding threshold signal in anadder.